The invention relates to a sheet forming fabric with reinforced margins.
In papermaking machines sheet forming fabrics are used for dehydrating the paper pulp. The sheet forming fabrics have a limited service life because on the backing side they are worn by friction against stationary dehydrating means of the papermaking machine. Such wear is enhanced by abrasive fillers which are added in a proportion of 20 to 22% by weight to the paper pulp in the manufacture of printing paper. Initially the sheet forming fabrics are worn only locally in the marginal zones. Frequently sheet forming fabrics show wear in the main ares of only 25 to 30% while in the marginal regions a narrow strip of 2 to 5 cm width is already completely worn through so that the fabric must be removed from the papermaking machine. These strips which are subject to especially high wear are located a distance of 6 to 18 cm inwardly from the edge of the sheet forming fabric. The cause for such locally concentrated high wear has not yet been completely discovered. In papermaking machines of conventional construction, i.e., those using a sheet forming Fourdrinier fabric, it is presumed that the cause for increased wear resides in the geometry of the openings of the suction boxes, in the vacuum distribution across the fabric width, and the like. Twin wire formers manufactured according to the most recent construction principles are seldom equipped with stationary suction boxes but exhibit the same unfavorable wear phenomena with highly concentrated wear along the marginal zones.
Various ways have been devised to prevent such unequal wear. From U.S. Pat. No. 4,011,131, it is known to provide a slot along the front edge of the suction boxes through which lubricant fluid, e.g., water, can be introduced into the friction area between the sheet former and the suction box. However, this method was not successful since, due to the high contact pressure in the region of the suction boxes, the water does not get into the marginal zones that are subject to wear. Also, relatively small amounts of water cannot prevent wear caused by the nearly ever present quartz particles.
U.S. Pats. Nos. 2,523,867, and 3,652,390, propose to adhere highly tenacious longitudinal filaments embedded into a synthetic resin composition or to fill the longitudinal strip along the marginal regions with a synthetic resin composition. However, this renders the marginal regions impervious, so that in these regions the pulp is not dehydrated. The pulp is then thrown off at the egoutteur or dandy roller, so that efficient quality production becomes impossible.
From German Offenlegungsschrift No. 2,922,025, it is known to stitch the fabric in the longitudinal direction in the marginal regions susceptible to wear on the backing side with an abrasion resistant yarn. However, it was found that the portion of the yarn disposed inside the fabric is not effective whereas the portion of the swing yarn disposed outside the fabric is soon worn through due to the high friction.
Moreover, sheet forming fabrics are known in which additional thick longitudinal threads are interwoven along the margins during weaving of said fabrics to increase the abrasion resistance. Said longitudinal threads have long floatings and a greater diameter than the structural threads of the fabric. Initially the marginal region of the sheet forming fabric slides on said additional longitudinal threads. However, since they protrude from the fabric plane as individual threads, they are worn after a relatively short time so that the fabric proper soon comes into contact with the machine parts. At the sites where said additional thick longitudinal threads are woven into the structure of the sheet forming fabric the transverse threads are deformed far outside the normal fabric plane owing to the unusually large diameter of the additional longitudinal threads, so that they, too, are soon worn through. After said transverse threads are worn through the marginal zone of the sheet forming fabric is very quickly destroyed.